This invention relates to edible, single-cell oil containing docosahexaenoic acid (DHA). The invention also relates to methods of producing such oil containing DHA in commercially viable yields and to products containing the oil.
DHA is an omega-3-fatty acid and is the most abundant long chain polyunsaturated fatty acid (PUFA) in the grey matter of the brain. Omega-3-fatty acids in general are known to be beneficial in reducing the incidence of coronary heart disease [Lands, Fish and Human Health (1986) Academic Press]. However, the metabolism of omega-3-fatty acids is not well understood. Thus, precise clinical dosages and efficacy remain unknown.
Cold water marine fish are a known source of omega-3-fatty acids, including DHA. U.S. Pat. No. 4,670,285 discloses the use of fish oil from fish such as menhaden and herring as a source of C.sub.2 omega-3-fatty acids. Indeed, fish oils are the primary commercial source of omega-3-fatty acids. Often, however, fish oils are unusable for human consumption because of contamination with environmental pollutants such as PCB's.
There also are problems associated with the recovery of fish oils containing DHA for food uses. Such oils often have a fishy odor and unpleasant tastes associated with the oxidation products of the fatty acids. These tastes and toxicities of peroxides render the oils unsatisfactory for use in edible compositions such as baby food and infant formulas.
Marine microorganisms also are known to contain DHA. In particular, various species of dinoflagellates are known to contain DHA. Harrington et al., "The Polyunsaturated Fatty Acids of Marine Dinoflagellates" J. Protozoal, 17:213-219 (1970), characterize the fatty acid content of eight photosynthetic and one heterotrophic marine dinoflagellates, and conclude that the dinoflagellates are a primary producer group of docosahexaenoic acid and contribute substantial amounts of that compound to the marine food chain.
Successful cultivation of dinoflagellates to produce an edible oil containing DHA has not been achieved. Dinoflagellates in general are very slow growing and are shear sensitive. Guillard et al., Dinoflagellates, (1984) Academic Press. The prior art discloses that even a small amount of agitation in the culturing vessel reduces growth of the cultures. However, such agitation would be necessary to achieve adequate oxygenation in order to maximize growth for commercial production.
DHA is thought to be essential for the proper brain and vision development of infants because, as noted above, it is the most abundant long chain PUFA in the brain and retina. Although a metabolic pathway exists in meals for the biosynthesis of DHA from dietary linolenic acid, this pathway is bioenergetically unfavorable [Crawford, P. AOCS. Short Course in Polyunsaturated Fatty Acids and Eicosanoids, pp. 270-295 (1987)] and mammals, like fish, are thought to obtain most of their DHA from dietary sources. In the case of infants, the most likely source would be human milk. Indeed, DHA is the most abundant C22 omega-3 PUFA in human milk. Generally, however, DHA is absent from infant formulas. U.S. Pat. No. 4,670,285 does disclose an infant formula containing omega-3-fatty acids. However, the acids utilized therein are obtained from egg or fish (Talapia) oil and have associated therewith the unpleasant characteristics previously described. Furthermore, fish oils generally contain another omega-3-fatty acid, eicosapentaenoic acid (EPA), an undesirable component in infant formulas because of its prolonged anticoagulant effects and its depression of arachidonic levels in infants. This has been correlated with reduced rates of infant weight gain (Carleson et al. INFORM 1:306.) Indeed, EPA levels are very low in human milk (less than one-forth that of DHA).
Accordingly, it is an object of the present invention to provide a single-cell edible oil containing DHA. Preferably this oil will have no significant quantities of other polyunsaturated fatty acids (PUFA's), i.e. greater than about 2% of the total fatty acid content. In general, it is an object of the present invention to produce single-cell oil in commercially viable yields. The oil, characterized herein as a "designer" oil, after extraction can be used in infant formulas, baby foods, dietary supplements and pharmaceuticals.
In addition, it would be desirable to acquire further knowledge of the metabolic pathway of omega-3-fatty acids. Isotopically labeled DHA would be of great utility in this regard. However, to date, no method has been known to produce abundant quantities of isotopically labeled DHA. Thus, it also is an object of the present invention to provide isotopically labeled DHA in sufficient quantities to undertake such research.